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Wireless Networks Thread, Network Link down until switch on other side of network restarted in Technical; Networks are silly. Problem: Link between two switches not working Solution: Restart a switch unconnected to either of the problematic ...
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    Question Network Link down until switch on other side of network restarted

    Networks are silly.
    Problem: Link between two switches not working
    Solution: Restart a switch unconnected to either of the problematic switches

    The problem was apparently a one off, but I'd be interested if anyone has any idea what was happening. We have switches A to E connected like this by Gigabit ethernet (as well as some other switches branching off):
    A-B-C-D-E
    Links are copper, except for a fibre link from C to D. With everything plugged in, no device that was connected to B, C, D or E could talk to any device connected to A, or vice versa. However if I unplugged cable B-C or cable C-D, then suddenly the link A-B works again. Unplugging D-E, did not let A-B work, so the problem was narrowed down to switch D. After I restarted switch D, the link A-B worked fine again.

    The switches were probably in a weird state from some network trouble earlier in the day: a computer had been continually spamming DHCP requests which caused the whole network to grind to a halt. However that had been stopped hours before - the A-B link problem appeared after the end of the school day when network traffic should have been reaching a minimum.

    Has anyone seen anything like this before? Does anyone have any idea how it could happen? My best guess is something to do with Spanning Tree Protocol (which we have switched on, though I do not think there is any need for it to be on).

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    did you set a root bridge on your stp setup? lack of a route bridge could explain what is happening.

    Ideally you shouldn't daisy chain 5 switches in a row. Try and keep it to 3 or 4 at a maximum.
    It would also be worth checking the switches for errors; boot them at the cli looking at the serial output POST for anything obvious. sometimes they boot ok but the post might reveal problems.

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    Jollity (5th December 2011)

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    No there is no root bridge set. The spanning tree settings were left on their defaults. I will have a look tomorrow to see what they are each set to. Though is there any reason to have spanning tree enabled at all if the topology of your switch connections is already a tree?

    I am aware the daisy chain is less than ideal. We have plans to get some more fibre laid so that we can get closer to a star, but these are waiting for the trenches to be dug.

    I have never had a look at the switch boot up using CLI. Would certainly be interesting to do so.

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    Spanning tree is generally a good idea, a slight misconfiguration or a cable looping a switch (typically if a student plugs in a cable between two network sockets) can bring down a significant portion of a network without it.

    You should definitely select the root bridge by giving it a lower priority than the others. This is usually your core switch. If you don't do this then the switch with the lowest mac address will act as the root, which can cause issues itself. You should also tell the edge ports on the switches that they are edge ports (i.e. if they don't have switches plugged into them, set stp edge-port enable)

    booting using a cli is a good tool. most switches have a serial port on them and you can connect using minicom, hyperterminal or putty usually on 9600 8n1 or 19200 8n1.

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    We had a problem with our switches on the network, it was different versions of stp on the network. We disable all stp and we found a loop. When we removed this all problems were solved no more restarting switches....

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    It is a strange problem. I had to read your post several times to get my head round it!

    What make are the switches?

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    Quote Originally Posted by CyberNerd View Post
    Spanning tree is generally a good idea, a slight misconfiguration or a cable looping a switch (typically if a student plugs in a cable between two network sockets) can bring down a significant portion of a network without it.
    Point taken: accidents do happen. I will leave it enabled, but try and get it properly configured.

    Quote Originally Posted by CyberNerd View Post
    booting using a cli is a good tool. most switches have a serial port on them and you can connect using minicom, hyperterminal or putty usually on 9600 8n1 or 19200 8n1.
    I got Putty set up and had a look at the CLI of one of our switches. It seems odd that terminal access still relies on a serial port, but I suppose if the system works there is too much inertia for a change.

    Quote Originally Posted by CyberNerd View Post
    You should definitely select the root bridge by giving it a lower priority than the others. This is usually your core switch. If you don't do this then the switch with the lowest mac address will act as the root, which can cause issues itself. You should also tell the edge ports on the switches that they are edge ports (i.e. if they don't have switches plugged into them, set stp edge-port enable)
    This sounds like an excellent plan. Presumably the idea of setting edge ports is then the switch will not send STP packets to them?

    I did not get on to sorting out the spanning tree settings this week. When I started looking at the switch configurations I noticed another problem: the QoS we are supposed to have for the VoIP phone system has never actually been configured. I do not see how this could directly cause the weirdness described above, but it might make the phones system rather less liable to fall over when there is network trouble. I definitely need to spend some quality time with our switches.

    Quote Originally Posted by Michael View Post
    It is a strange problem. I had to read your post several times to get my head round it!
    It certainly made my head spin. Though even if it were not so strange, I would need several readings to make sense of my paragraph of alphabet soup.

    Quote Originally Posted by Michael View Post
    What make are the switches?
    All 3Com. B and D are the somewhat elderly Superstack 3 Switch 4200 model. A, C, E are newer Baseline switches.

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